Here's my perspective: my big heavy trainer (which is quite acrobatic incidentally) was fitted with an OS40 LA and variously 11x4 or 11x5 prop. fine in air, rubbish on ground when grass anything other than shaved. I fitted an ASP 52FS (magnum or SC in the US) with a 11x6 prop and the thing just leaps off the ground. it's great. Now, the os40 produces 1.0 hp the 52FS produces 0.9 but the torque is completely different. Don't get me wrong, I love the simplicity of my little OS 40 but love the sound and the grunt of my 52. I think some of the other correspondents have suggested a 70fs (or 80) in your p51, that would sound great and if your kit asks for a 46 it would do the job...